Electroacoustic devices include headphones, headsets, helmets, speaker enclosures and other devices having electro-acoustic functions. Headphones typically include a pair of ear cups mounted on respective ends of an arcuate or C-shaped adjustable headband. Each of the ear cups contains a headphone speaker that converts electrical energy from a television, radio, compact disk (CD), cassette tape or the like into acoustic energy that is perceived by the ears of the wearer. Headsets additionally include a “boom” microphone that is positioned in proximity to the wearer's mouth to permit the wearer to engage in two-way communication with a second person. Most headphones and headsets include some type of noise reduction capability which reduces the quantity of unwanted acoustic energy that reaches the ears of the wearer.
There are two general types of noise-reducing or noise-canceling capability among headphones and headsets. The most basic of these capability types is passive noise attenuation, in which the physical structure of the headphone insulates the wearer's ears from extraneous and unwanted noise. Headphones and headsets characterized by passive noise attenuation may include an acoustically-absorbent material which lines the interior of the ear cups, as well as some form of an ear cushion that lines the edge of each ear cup and presses against the wearer's skin around the ear during use. The second type of noise-canceling capability is known as active noise attenuation and requires an electromechanical device and electronic circuitry. This type of noise-canceling capability results from a combination of active noise attenuation and passive noise attenuation.
Those headphones and headsets having active noise attenuation capability require a microphone or microphones to pick up the original sound and convert this original sound to electrical energy, electronic circuitry to control the electrical energy of the original sound and a speaker to convert the electrical signal back to an acoustic signal. The electronic circuitry inverts the phase of the original sound by 180 degrees and amplifies the signal to an acoustic level which is equal to the level that reaches the wearer's ear. The amplified signal, 180 degrees out of phase with respect to the original signal, cancels the original signal and results in a clearer sound perceived by the ear of the headphone or headset wearer.
One of the problems inherent in conventional active noise reduction designs for headphones is that the microphone or microphones and the speaker used in the headphone have sensitivities, frequency responses and phase responses which differ from each other. Furthermore, the acoustic time delay between the microphone and the speaker causes a phase shift between the original signal and the attenuated signal. Consequently, not all of the frequencies in the original signal will be canceled by the attenuated signal because not all frequencies of the attenuated signal will be 180 degrees out of phase with respect to all frequencies of the original signal. It has been found that placement of a microphone and a speaker in substantially the same acoustic plane minimizes the difference between the sound wave phase or time delay and the sound pressure that acts on the microphone diaphragm with respect to the signal from the speaker.
Another problem that exists in active noise reduction designs is that the microphone or microphones tend to pick up direct reflections of certain frequencies of the original signal. This distorts reproduction of the phase-inverted original signal as the amplified attenuated signal. Accordingly, new and improved electroacoustic devices are needed which are capable of minimizing adverse noise reduction effects associated with the differences in sensitivities, frequency responses and phase responses and acoustic time delays which exist between microphones and speakers, as well as minimizing direct sound reflections picked up by the microphone or microphones.